Unfortunately I'm not sure how the Canna and GH ratios would compare; what I'd do is start at a rather low dose of the Ca/Mg additives (so maybe half of what the bottle says to start), watching the low leaves for signs of Ca def (yellow with necrotic light brown blotches) and the middle leaves for Mg def (clearly-defined pale stripes between the veins). Adjust your nutes accordingly if you see this. Be careful not to go too high too fast, especially with the Mg supplement.